1. Field of the Invention
The present invention relates to an objective image quality assessment device of a video quality for assessing the quality of an image, degraded as a result of compression coding a video image, with only the compressed bit stream information without using the baseband information of a reference image and a decoded image, and an automatic monitoring device using the same.
2. Description of the Related Art
The amount of information is normally reduced through compression coding when storing and transmitting digital images. The compression coding referred to herein generally means an irreversible compression. The irreversible compression is a compression form of reducing the amount of information under the condition of sufficiently suppressing the visual degradation, that is, maintaining the image quality sufficiently high without completely rebuilding the original image when decoding the coded information (encoded bit stream). A typical example of the irreversible compression includes MPEG-2 and H.264 (non-patent documents 1, 2).
In the irreversible compression, the coding is carried out with the visual degradation sufficiently suppressed, as described above, but the degradation becomes visually recognized as the compression rate becomes high, that is, as the bit rate lowers. The irreversible compression also has a characteristic in that the extent of degradation that is visually recognized differs depending on the characteristics of the video such as fineness of the object, velocity and complicatedness of the movement, and the like in the screen even if the compression rate is the same. Thus, a technique of quantitatively measuring the degradation in image quality involved in the irreversible compression is desired.
The conventional image quality measurement has been carried out through a method called a subjective assessment. In this method, about twenty subjects are gathered and a video is presented to the subjects, where marks are given according to the subjective view of the subjects and the numerical value (e.g., average of mark) obtained by statistically processing such marks is defined as the quality of the video. A representative method of the subjective assessment method is defined in ITU-R recommendation BT.500-11, ITU-T recommendation P 910 and the like (non-patent documents 3, 4). However, the subjective assessment is not an easy method for assessing the video quality as the strict viewing conditions defined in the recommendation need to be satisfied and a great number of subjects need to be recruited.
Therefore, an object image quality assessment of extracting one or a plurality of numerical indices indicating the characteristic of the video called the video feature by analyzing the video signal, and deriving the quality of the video from the video feature is being reviewed. The image quality derived by the objective image quality assessment presumes the subjective image quality, and aims to be used as a substitute of the subjective image quality assessment.
In ITU-T J.143 (non-patent document 5), the framework of the objective image quality assessment method is defined. The framework of the objective assessment method is classified into the following three depending on the video of which stage, transmission or storage, to use for the assessment.
(1) Full Reference (FR) type: a method for using the baseband information of the original image before the compression coding and the decoded image (in case of storage), or the transmission image and the reception image (in the case of transmission).
(2) No Reference (NR) type: a method for using only the baseband information of the decoded image or the reception image (information of the original image or the transmission image is not used).
(3) Reduced Reference (RR) type: a method for using the image feature of the original image or the transmission image in which the amount of information is limited, and the baseband information of the decoded image or the reception image.
The Full Reference type has the highest estimation accuracy of the subjective image quality of the three frame works since the baseband image of before and after the storage or the transmission can be used. The No Reference type is inferior to the Full Reference in terms of accuracy since only the baseband image of after storage or transmission is used. The Reduced Reference type uses the image feature of the original image or the transmission image in addition to the baseband information of the decoded image or the reception image used in the No Reference type. The image feature is about a few dozen to a few hundred kbps, and is limited to a sufficiently small amount of information compared to the baseband information of the original image. In the RR type, the image feature on the transmission side is transmitted to the reception side using the data line prepared separate from the video line in time of video transmission in an aim of enhancing the estimation accuracy of the subjective image quality to higher than the NR type.
The objective assessment method based on the FR type among the three types of frame works includes the ITU-T recommendation J. 144 (non-patent document 6), the ITU-T recommendation J.247 (non-patent document 7), and Japanese Patent Application Laid-Open No. 2008-35357 (patent document 1). Non-patent document 6 shows the objective image quality assessment method targeting on the coding degradation of the standard television method (SDTV), and non-patent document 7 and patent document 1 show the objective image quality assessment method targeting on the video format often used in the multimedia application.
The ITU-T recommendation J.246 (non-patent document 8) is known for the objective assessment method based on the RR type. Non-patent document 8 discloses the objective assessment method based on the premise of the video format of the multimedia application.
Non-patent document 1: ITU-T Recommendation H.262, “Information technology-Generic coding of moving pictures and associated audio information: Video”
Non-patent document 2: ITU-T Recommendation H.264, “Advanced video coding for generic audiovisual services”
Non-patent document 3: Recommendation ITU-R BT. 500-11, “Methodology for the subjective assessment of the quality of television pictures”
Non-patent document 4: ITU-T Recommendation P.910, “Subjective video quality assessment methods for multimedia applications”
Non-patent document 5: ITU-T Recommendation J.143, “User requirements for objective perceptual video quality measurements in digital cable television”
Non-patent document 6: ITU-T Recommendation J. 144, “Objective perceptual video quality measurement techniques for digital cable television in the presence of a full reference”
Non-patent document 7: ITU-T Recommendation J.247, “Objective perceptual multimedia video quality measurement in the presence of a full reference”
Non-patent document 8: ITU-T Recommendation J. 246, “Perceptual audiovisual quality measurement techniques for multimedia services over digital cable television network in the presence of a reduced bandwidth reference”
Patent document 1: Japanese Patent Application Laid-Open No. 2008-35357
Japanese Patent Application No. 2008-29359, which is a prior application of the applicant, is known for the objective assessment method by the NR type. The NR type image quality assessment of the invention of the prior application is inferior to the FR type in terms of estimation accuracy of the subjective image quality since it does not use the information of the original image, but has an advantage in that the system configuration is simple since assessment with only the decoded image/reception image is possible. Such method is thus an effective method in the application of the transmission video monitoring.
In the invention of the prior application, the objective image quality is estimated by extracting temporal and spatial feature through analysis of the baseband signal of the decoded image, and integrating the same. The baseband signal enables fine analysis since the information in units of pixels can be acquired, but requires an enormous resource for the reading of the non-compressed signal having an enormous amount of information, storage to the memory, and the calculation process thereof. Therefore, the amount of information to be analyzed needs to be reduced, and consequently, the resources for calculation and information storage need to be reduced.